Two oxygenated iron-cobalt hybrid hemoglobins (Hbs), (Co-O2Fe-O2)2 and Fe-O2(Co-O2)2, were studied by electron spin echo envelope modulation (ESEEM) spectroscopy in order to measure (i) electron-nuclear hyperfine and nuclear quadrupole coupling to the Nn of the proximal histidyl imidazole and ii) nuclear hyperfine coupling to exchangeable 2H in the oxyCo subunits. 14N couplings were found to be smaller in the oxyCo-subunits than in the oxyCo-subunits, suggesting a more ionic and shorter Co-O2 bond in the subunits which correlates with the higher O2 affinity found for Co(Fe-O2)2 Hb than for (Fe-O2-Co)2 Hb. A smaller nuclear quadrupole coupling constant found for the proximal histidyl Nn sp2 hybrid and the Co dd2 orbital, i.e., a shorter Co-Nn bond, than in the oxyCo ~ subunits. A hyperfine coupled 2H was detected in the spectra of D2O-exchanged samples of both hybrid Hbs, supporting the presence of a hydrogen bond to bound O2 in both oxyCo and subunits.